The present invention relates generally to methods and apparatus for measuring deviations from flatness on a horizontal surface, and more particularly to methods and apparatus for doing so with improved speed and accuracy.
There are a number of instances where the flatness of a horizontal surface is a matter of critical importance. Examples of such instances include floors in warehouses having high storage racks. In warehouses of this type, goods are moved to and from the storage racks by fork-lift trucks having lifting forks mounted for vertical movement on vertical frames extending as high as 40 feet above the floor, for example. As these trucks move across the floor, deviations in floor flatness are manifest by vibrations or oscillations in the vertical frame, and the amplitude of these oscillations increases with the height of the frame. These oscillations are undesirable, and accordingly, flatness tolerances on the floors in such warehouses are extremely strict. If the flatness deviations are outside these tolerances, the floor must be refinished to bring flatness within the tolerances.
Flatness deviations comprise bumps and recesses, and these are defined by (a) slope (i.e. the angle from the horizontal), (b) vertical displacement from a given reference level and (c) curvature (change of slope). In order to determine whether floor flatness is within tolerance, it is necessary to measure the flatness of the entire floor, and this requires a multiplicity of flatness measurements at relatively closely spaced intervals in two mutually perpendicular directions and encompassing the entire area of the floor. There are a number of types of measuring devices previously employed for this purpose, but they all have drawbacks of one type or another.
There are manual systems for measuring flatness deviations, and these usually employ a traditional engineer's optical level and rod or a level straight edge with a sliding dial gage mounted at right angles to the straight edge. Measurements obtained from these manual systems are then graphically plotted on a grid of the floor. These systems are tedious, labor intensive and often require skilled personnel. Another drawback is that the time constraints associated with these manual systems limit the number of actual measurements to a relatively small number of points on the floor, and this requires interpolation from the actual measurements to reflect flatness deviations between the measurement points. Interpolation does not necessarily provide a true indication of flatness deviations between measurement points. In floors requiring the critical flatness characteristics under discussion here, such interpolations are not acceptable.
Reducing the interval at which the above-described manual measurements are made, to the extent necessary to avoid unacceptable amounts of interpolation, makes the entire measurement job extremely lengthy, tedious and expensive.
Another manual method for measuring flatness deviations employs a device which is moved across the floor along a line on a step by step basis. At each step an instrument mounted on the device gives a reading of slope or displacement. This procedure is repeated along a multiplicity of spaced lines on the floor, and the readings at each step on each line are recorded and plotted. This procedure too is tedious and time consuming.
Other flatness measurement devices employ wheeled vehicles mounting instruments which measure floor slope as the wheeled vehicle moves across the floor along a predetermined line. This procedure is repeated along a multiplicity of spaced lines. Measurements obtained from the slope sensing device can be recorded and plotted. Although this procedure is less tedious and less labor intensive than the manual procedures described above, there are distortions in the data produced by such a procedure in that the slope measurement produced for a given floor location may not accurately reflect the actual slope at that location, and the curvature and displacement data provided by such a procedure are also not accurate.